In recent publication on citrus hybrid origin based on DNA markers it was concluded that:

"Thus, kabuchi was inferred to be an offspring of kunenbo-A as seed parent and an unidentified variety, andkeraji was inferred to be an offspring of kabuchi and kunenbo-A, but their combination was indeterminate.This inferred parentage suggests that keraji is a backcrossed offspring of kunenbo-A."

Radoslav,That is exactly what is claimed in DNA paper: Kabuchi is a hybrid between KunenboA and and unknown citrus variety with Keraji being its backcross to Kunenbo (KunenboA X Kabuchi).For morphological differences- I just sent you by email two original papers with descriptions made by T.Tanaka